Owing to a significant growth in regenerative energy generating units in the supply grids, there are consistently increasing minimum requirements that the grid operators set for all feed-in units as regards stability and supply safety. In this regard, there are grid codes which demand a uniform course of action. Thus, for example, in the Finnish grid code “Fin-Grid”, fault clearing times of 250 ms at 0 p.u. residual voltages are required, for example, which for some generators, in particular synchronous generators, results in a fall into asynchronicity, without additional measures, and therefore in a certain resynchronization after recovery of the mains voltage. FIG. 1 shows several grid code requirements. The abovementioned “Fin-Grid” is provided with the reference symbol 1. Further grid code requirements are illustrated here by way of example for E. on 2, REE Spain 3 and WECC North America 4.
The course of action for resynchronization after recovery of mains voltage can take a few minutes, during which the power plant power is not available to the grid. This can primarily result in grid instability and in the worst case scenario in a large-area voltage drop in the case of failure of relatively large power plants.
During the short circuit, the mechanical power impressed onto the shaft assembly by the turbine is no longer taken off at the generator, and therefore results in acceleration of the turbo set.
If the rotor angle of the synchronous generator exceeds a critical transient value, said synchronous generator falls into asynchronicity and needs to be resynchronized. It is demanded in the grid codes that a power plant needs to be able to run through a predefined fault clearing time at a certain residual voltage on the transformer high-voltage side without grid isolation. If this required fault clearing time is above the fault clearing time which can be achieved for the turbo set, additional precautions need to be taken.
Several possibilities for taking into consideration this circumstance are known from the prior art. Thus, for example, in EP 1 805 887 B1, an active boost circuit is connected in series with the generator field winding via slip rings on a charged capacitor in the event of a fault, as a result of which the field voltage is raised suddenly. Thus, the generator is in the over excited range in the event of grid recovery, as a result of which the stability of the turbo set/grid system is increased.
A further possibility for extending the critical fault clearing time includes increasing the moment of inertia of the assembly in order to reduce the shaft acceleration in the event of a short circuit.
Furthermore, for some types of turbine there is the possibility of making changes to the steam turbine in order to clear the steam away from the turbine blades even more quickly, which is referred to as fast valving. Thus, a quicker reduction in the turbine power impressed on the shaft assembly is intended to be achieved.
Likewise, it is known to form similar concepts with loading resistors in the case of offshore wind turbines. However, wind farms are connected to an onshore converter plant via high-voltage DC connections, for example, said onshore converter plant diverting the excess energy from the wind farm into loading resistors in the event of a short circuit on the land.
It would be desirable to have a simple possibility for a turbo set for extending the fault clearing times in the event of a short circuit.